A random access channel (RACH) is a channel used for transmitting a short message over one or two frames in uplink. The channel structure of the RACH and the RACH access process are disclosed in the 25.211 and 25.214 of the Technical Specification (TS) of the Third Generation Partnership Project (3GPP).
The RACH is an uplink transmission channel in which signals are always received from the entire cell. The RACH features a collision risk and an open loop power control.
Data packets of a medium length around 50 frames at largest are transmitted through a common packet channel (CPCH), and data of over 50 frames such as voice data are transmitted through a dedicated channel.
In case of a terrestrial mobile communication system for International Mobile Telecommunication-2000 (IMT-2000), a mobile station transmits a preamble to an access network through a random access channel (RACH) before sending out a message.
If any acquisition indicator signal of the transmitted preamble does not arrive from the access network within a predetermined time, the mobile station increases transmission power, retransmits the preamble and waits for the acquisition indicator signal from the access network again.
When the mobile station that has transmitted the preamble to the access network receives the acquisition indicator signal from the network within the predetermined period, it finally sends out the message. In short, an acquisition indication procedure for the preamble reception is performed before a message is transmitted.
Also, it can be checked out by the mobile station whether or not the transmitted message is received in the access network without error after receiving a response for the message—not an acquisition indicator signal of the message but a response signal of the message content—from the access network. To transmit the response to the message to the mobile station, it should be processed in the upper layers of the access network and it takes time to do it.
There are a couple of problems and requirements to apply the RACH access method of the terrestrial mobile communication system to a satellite mobile communication system.
First, since propagation delay time in the link between a mobile station and an access network is generally less than 1 ms in the terrestrial mobile communication system, the waiting time for an acquisition indicator signal after transmitting a preamble is short. Therefore, in the terrestrial mobile communication system, a preamble is sent out prior to a message, and after acknowledging the preamble, the message is transmitted. This way, the probability for a successful message transmission can be heightened.
However, in a satellite mobile communication system, the propagation delay time in the link between a mobile station and a satellite access network is more than decades or hundreds of ms, and naturally the waiting time for an acknowledge signal after the transmission of a preamble is several times of the propagation delay time. Accordingly, when the RACH access method of the terrestrial mobile communication system is applied to the satellite mobile communication system, it takes severely long time to transmit a preamble and acquire the indication on the successful reception of the preamble because of long propagation delay time. As a result, there is a problem that the message transmission delay becomes very large.
Secondly, the distance between a satellite or an earth station of the satellite access network and a mobile station in the satellite mobile communication system is much more distant than that between a base station and a mobile station in the terrestrial mobile communication system, i.e., the propagation delay is long, and thus the received power of the preamble is relatively smaller. Therefore, the probability of the successful preamble reception at the satellite access network side is very low. In addition, in case of using a low earth orbit satellite, the Doppler shift effect due to satellite movement occurs and reaches as far as tens of kHz. Therefore, in a satellite mobile communication system, there is a problem that much energy should be assigned to the preamble in order to enhance the reception probability in case of transmitting a single preamble as in the terrestrial mobile communication system.
Thirdly, in the terrestrial mobile communication system, the problem of message transmission delay is not severe even when the procedures of transmitting a preamble prior to a message, confirming the acquisition of the preamble, transmitting a message and receiving a response to the message—which is not an acquisition indicator signal for whether the message is received, but for the content of the message—are performed, because the link delay time between the mobile station and the base station is short.
Also, for the terrestrial mobile communication system, although the preamble and the message are transmitted together in the conventional ALOHA protocol and the response to the message is received without an acquisition indicator (AI) signal for the preamble acquisition, the message transmission delay time including the time for processing a response to a message in the upper layers in the access network does not cause any problem, thanks to short propagation delay time between the mobile station and the base station. In case of making access to the RACH in the ALOHA protocol, the mobile station transmits a message together with its preamble and knows whether the message and the preamble are successfully received at the base station without error by receiving a response to the transmitted message. Therefore, a processing time in the upper layers of the access network is required including the access network to process the response to the message from the mobile station and transmit the response to the mobile station. In short, only after the time for signaling and processing in the upper layers, which are necessary inside the network, passes, a mobile station can receive the response to the message and confirm if the message is received without error.
Therefore, in the terrestrial mobile communication system, although the message transmission delay includes the time for signaling and processing a response to the message in the upper layers, the time delayed until the mobile station receives the response to the message does not become a big problem.
However, as described above, in the satellite mobile communication system, the propagation delay time reaches tens or hundreds of ms, and the waiting time for the response to a message after the message transmission is several times of the propagation delay time. Therefore, it takes seriously long time until the preamble is acquired, the successful reception of the preamble is confirmed, and the response to the message is received.
Further, although it may be different according to environments of the mobile communication system, in general, the time for signaling and processing a response to a message is larger than the link propagation delay time. Accordingly, the link propagation delay, which is negligible in the terrestrial mobile communication system, is severe in the satellite mobile communication system.
Fourthly, in case of a mobile communication system using a slotted RACH method, where a mobile station transmits a packet through the RACH to be received in the access network within a slot, the mobile station should carefully control the packet transmission time to stay within the precision of the slot of the access network. To synchronize the reception time of a packet with a slot at the access network, the propagation delay time between the mobile station and the access network should be figured out precisely and the packet transmission time should be controlled. Therefore, before the packet is transmitted through the RACH, a transmission for slot synchronization and a feedback procedure for the synchronization between the mobile station and the access network should be performed, or the exact propagation delay time should be figured out by using a signal from an external device such as a global positioning system (GPS) and confirming the exact location of the mobile station and the satellite.
For these reason, in a satellite mobile communication system, it is preferred to simplify the synchronization of the RACH.
Fifthly, as mentioned above, since the received power of the packets at the satellite access network is deteriorated, when two mobile stations close to each other transmit packets at the same time, the satellite access network receives the packets almost simultaneously, such that the interference to each other is increased and the packet reception probability is seriously dropped. When packets transmitted from a plurality of mobile stations simultaneously are received by a satellite access network, the reception times of the packets at the satellite access network are centralized into a particular time duration, which depends on the difference of round trip delay times. Therefore, when the round trip delay time difference is very small, the reception time of packets from the mobile stations are centralized in a particular time, such that the interference to each other is increased and the packet reception probability is seriously dropped.
Finally, in case of the terrestrial mobile communication system, after transmitting a message, the mobile station will waits for a response to the message from the access network during a predetermined time, and if the access network doesn't receive the message successfully, there is a problem that it would take at least two round trip delays from the transmission time of the previous preamble for the mobile station to retransmit the preamble. This problem turns out to be more serious in the satellite mobile communication system, in which link delay time is much longer than that of the terrestrial mobile communication system.